Wouldn't the clear line be actually taking up arms against Ukraine? If he does a publicity stunt at a Russian army training base, that's an evil and stupid thing to do, but it's not actually helping the military effort against Ukraine. Now, if they trained him as a FPV attack drone pilot, and then showed on camera him piloting drones targeting Ukrainian troops? That would probably make him a legitimate target. At least that's what I would think. But my thought would be that the line would be actually taking up arms in some form against Ukrainian troops or civilians.